The invention concerns a method for the manufacture of very fine threads from melt-spinnable polymers and an apparatus for the manufacture thereof
Microthreads of this kind, but usually microfibres of finite length, have for many years been made by a hot-air spun-blown method, the so-called melt-blown method, and today there are different apparatuses for this. A common feature of all of them is that, in addition to a row of melt holesxe2x80x94several rows parallel to each other have also become knownxe2x80x94hot air which draws the threads escapes. By mixing with the colder ambient air, there is cooling and solidification of these threads or fibres, for often, usually of course undesirably, the threads break. The disadvantage of this melt-blown method is the high expenditure of energy to heat the hot air flowing at high speed, a limited throughput through the individual spin holes (even though these have been set increasingly closer together in the course of time, down to a spacing of below 0.6 mm with 0.25 mm in hole diameter), that with thread diameters of less than 3 xcexcm breaks occur, which leads to beads and protruding fibres in the subsequent textile composite, and that due to the high air temperature necessary to produce fine threads the polymers are thermally damaged well above the melt temperature. The spinning nozzles, of which a large number have been proposed and also protected, are elaborate injection moulding dies which have to be made with high precision. They are expensive, operationally susceptible and tedious to clean.
It is therefore the object of the present invention to provide a method and an apparatus for the manufacture of essentially endless threads, which require less expenditure of energy, do not cause thread damage on account of excessive temperatures and use a spinning tool of simple construction.
This object is achieved according to the invention by the characteristics of the independent claims.
The present invention avoids the disadvantages of the state of the art by the fact that polymer melt is pressed out of spin holes, which are arranged in one or more parallel rows or rings, into a chamber having a given pressure which is filled with gas, as a rule with air, and which is separate from the environment, wherein in the molten state the threads pass into a region with rapid acceleration of this gas at the outlet from the chamber. The forces transmitted to the respective thread on the way there by shear stress increase, its diameter decreases greatly and the pressure in its still liquid interior increases to a corresponding extent in inverse proportion to its radius due to the effect of the surface tension. Due to the acceleration of the gas, its pressure drops by the laws of flow mechanics. In the process, the conditions of the melt temperature, gas flow and its rapid acceleration are coordinated with each other in such a way that the thread before solidification thereof attains a hydrostatic pressure in its interior which is greater than the surrounding gas pressure, so that the thread bursts and divides into a plurality of fine threads. Due to a gap at the bottom in the chamber, threads and air leave the latter. Bursting takes place after the gap and under otherwise unchanged conditions with surprising stability at a given fixed location. In the region of great acceleration, gas and thread streams run parallel, the flow by interface around the threads being laminar. Continued splitting of the original thread monofilament occurs without bead formation and breaks. From a monofilament is produced a multifilament of very much finer threads using a gas stream having ambient temperature or gas stream slightly above this.
The new threads arising from splitting are considerably finer than the original monofilament. They may even still be drawn slightly after the splitting point until they are solidified. This happens very quickly because of the greater thread area suddenly created. The threads are endless. But more to a minor extent they can be threads of finite length due to deviations in the polymer, individual speed or temperature disturbances, dust in the gas and the like disturbances in real industrial processes. The process of splitting thread-forming polymers can be adjusted in such a way that the numerous very much finer single filaments produced from the monofilament are endless. The threads have a diameter of well below 10 xcexcm, mainly between 1.5 and 5 xcexcm, which in the case of polymers corresponds to a titre of between about 0.02 and 0.2 dtex, and are referred to as microthreads.
The area of great acceleration and pressure drop in the gas stream is according to the invention realised in the form of a Laval nozzle with convergent contour to a narrowest cross-section and then rapid widening, the latter already so that the newly formed single threads running adjacent to each other cannot stick to the walls. In the narrowest cross-section, with a suitable choice of pressure in the chamber (in the case of air, about twice as high as the ambient pressure behind), the speed of sound can prevail, and in the wider portion of the Laval nozzle supersonic speed prevails.
For the manufacture of non-woven thread fabrics (spun-bonded fabrics), spinning nozzles in row form and Laval nozzles of rectangular cross-section are used. For the manufacture of yarns and for special kinds of non-woven fabric manufacture, round nozzles with one or more spin holes and rotationally symmetrical Laval nozzles can also be used.
The method borrows from methods for the manufacture of metal powders from melts, from which it was developed. According to DE 33 11 343, the molten metal monofilament in the region of the narrowest cross-section of a Laval nozzle bursts into a large number of particles which are deformed into pellets by the surface tension and cooled down. Here too the result is a liquid pressure in the interior of the melt monofilament which outweights the surrounding laminar gas flow. If the pressure drop takes place so rapidly that solidification is not yet close, the pressure forces can outweigh the forces of cohesion of the molten mass, mainly viscosity forces, and bursting into a plurality of filament pieces (ligaments) occurs. The crucial factor here is that the thread must remain liquid at least in the interior so that this mechanism can set in. It has therefore also been proposed to further heat the monofilament after its emergence from the spinning nozzle.
Automatic bursting of a molten metal thread is also named the xe2x80x9cNANOVAL effectxe2x80x9d after the firm which uses it.
Defibration by bursting has become known in the manufacture of mineral fibres, thus in DE-A-33 05 810. By interfering with the gas flow in a rectangular channel arranged below the spinning nozzle by means of fittings which generate cross flows, as stated there the result is defibration of the single melt monofilament. In a not quite clear account there is mention of defibration by static pressure gradient in the air flow, and in fact in EP 0 038 989 drawing from a xe2x80x98loop or zigzag movement . . . after the fashion of a multiple whiplash effectxe2x80x99. The fact that the actual xe2x80x98defibrationxe2x80x99 is caused by an increase in pressure in the interior of the thread and decrease in the surrounding gas flow was not recognised, nor any control mechanisms in this direction.
For polymers, this finding from mineral fibre manufacture was obviously made use of by the same applicant firm. In DE-A-38 10 596 in an apparatus according to FIG. 3 and description in example 4 the melt stream of polyphenylene sulphide (PPS) is xe2x80x98defibrated by a high static pressure gradientxe2x80x99. The gas streams are hot, even heated beyond the melting point of the PPS. A static pressure gradient in the gas flow, decreasing in the direction of thread travel, cannot on its own defibrate the thread. It was not recognised that, for this, the melt stream must remain liquid in its interior, at least in an adequate portion. But by using hot air in the region of the polymer melt temperature, this happens by itself. It is not a xe2x80x98pressure gradient occurring after the outlet holesxe2x80x99, column 1, lines 54-55 that draws the melt streams into fine fibres, but a static pressure gradient between melt stream and surrounding gas flow that causes it to split or defibrate. The threads produced are of finite length and amorphous.
The threads of the method according to the invention on the other hand are endless or essentially endless. They are produced by selectively controlled bursting of a still molten monofilament in a laminar gas flow surrounding them, that is, without turbulence-generating cross flows. Basically all thread-forming polymers are considered, such as polyolefins PP, PE, polyester PET, PBT, polyamides PA 6 and PA 66 and others such as polystyrene. Here, those such as polypropylene (PP) and polyethylene (PE) are to be regarded as favourable because surface tension and viscosity are in a ratio which readily allows the build-up of an internal thread pressure against the surface tension force of the thread skin, while the viscosity is not so high that bursting is prevented. The ratio of surface tension to viscosity can be increased by increasing the melt temperature in most polymers. This takes place in a simple manner in melt manufacture and can be reinforced by heating the spinning nozzles shortly before emergence of the threads. Heating the threads afterwards by hot gas streams does not however take place according to the present invention.
It can be established that the subject of the invention, controlled splitting of a polymer thread drawn with cold air into a plurality of finer single threads of endless or essentially endless single threads, has not yet been found. This takes place by the automatic effect of bursting of the melt thread due to a positive pressure difference between the hydraulic pressure in the thread, arising from the surface tension of the thread envelope, and the gas flow surrounding it. If the pressure difference is so great that the strength of the thread envelope is no longer sufficient to hold together the interior, then the thread bursts. Splitting into a plurality of finer threads occurs. The gas, usually air, can be cold, i.e. does not have to be heated, only the process conditions and the apparatus must be such that the melt monofilament in its critical diameter which depends on the melt viscosity and the surface tension of the polymer concerned is not cooled to such an extent that it can no longer burst due to the internal liquid pressure building up. Also the melt holes must not be cooled by the gas so greatly that the melt cools down too greatly, let alone already solidifies there. The process and geometrical conditions for producing this splitting effect are relatively easy to find.
The advantage of the present invention lies in that, in a simple and economical manner, very fine threads within a range of well below 10 xcexcm, mainly between 2 and 5 xcexcm, can be produced, which in the case of pure drawing for example by the melt-blow method can be accomplished only with hot gas (air) jets heated above melting point, and so requires considerably more energy. Moreover, the threads are not damaged in their molecular structure by excessive temperatures, which would lead to reduced strength, with the result that they can then often be rubbed out of a textile structure. Another advantage lies in that the threads are endless or quasi-endless and cannot protrude from a textile structure such as a non-woven fabric and come away as fuzz. The apparatus for carrying out the method according to the invention is simple. The spin holes of the spinning nozzle can be larger and so less susceptible to breakdowns, and the Laval nozzle cross-section in its precision does not need the narrow tolerances of the lateral air slots of the melt-blown method. For a given polymer one need only coordinate the melt temperature and the pressure in the chamber with each other, and with a given throughput per spin hole and the geometrical position of the spin holes relative to the Laval nozzle splitting occurs.